After I performed a Flash player downgrade on my MacBook because I couldn't see some games, I noticed (and I don't know if it is even related to the problem) that I can't type the @ anymore on firefox. When I do it, it takes me to the next open tab.

That's really strange. The keyboard shortcut in FF for next tab is Ctrl+Tab. There is no way to customize keyboard shortcuts in FF. So somehow FF is mistaking your Shift+2 for Ctrl+Tab. Try the Character Viewer/Punctuation and "type" an "@" using your mouse. Does that perform a next tab too?

I should've said I'm using a Spanish keyboard, thus, alt +2 should give me the @ character. If I do cmd+2 it takes me to the next tab. Sometimes when I type alt+2 it takes me to the next tab, and sometimes it doesn't do anything.

I tried using the screen keyboard and I get the same.

I also tried switching to another Spanish layout (such as LatinAmerican) but got the same results.